docs: def sr(x,filter=None, iface=None, nofilter=0, *args,**kargs):
      """Send and receive packets at layer 3
  nofilter: put 1 to avoid use of bpf filters
  retry:    if positive, how many times to resend unanswered packets
            if negative, how many times to retry when no more packets are answered
  timeout:  how much time to wait after the last packet has been sent
  verbose:  set verbosity level
  multi:    whether to accept multiple answers for the same stimulus
  filter:   provide a BPF filter
  iface:    listen answers only on the given interface"""
      if not "timeout" in kargs:
          kargs["timeout"] = -1
      s = conf.L3socket(filter=filter, iface=iface, nofilter=nofilter)
      a,b=sndrcv(s,x,*args,**kargs)
      s.close()
      return a,b